Long QT syndrome (LQTS) is a disorder characterized by delayed cardiac repolarization and an increased risk of arrhythmias and sudden death. Long QT syndrome type 2 (LQT2) is caused by mutations in the human ether-a-go-go-related gene (hERG). hERG encodes the pore-forming subunit of the rapidly activating delayed rectifier potassium channel in the heart. LQT2 is the second most prevalent form of LQTS, accounting for 35% to 40% of genotyped cases of LQTS. LQT2 mutations can cause hERG channel dysfunction by a variety of mechanisms. In the previous funding period, we have shown that nonsense-mediated mRNA decay and splicing defects are important mechanisms of hERG channel dysfunction in LQT2. We have also shown that generation of hERG C-terminal isoforms is determined by competition between alternative splicing and polyadenylation of hERG intron 9 and that the relative expression of hERG C-terminal isoforms plays an important role in regulation of hERG channel function. In the present application, we will use full-length hERG gene constructs to study mechanisms that underlie the regulation of hERG C-terminal isoform expression, characterize two new mechanisms of hERG channel dysfunction in LQT2, and develop a novel approach to modulate the relative expression of hERG C-terminal isoforms. In addition, we will use patient-specific induced pluripotent stem (iPS) cell-derived cardiomyocytes as a model to study pathophysiology of LQT2. The specific aims of this application are: Aim 1) To study a newly identified LQT2 splice site mutation that disrupts the 3'splice site of intron 9 and alters the relative expression of hERG C-terminal isoforms. Aim 2) To develop an antisense approach to increase hERG current by inducing a shift in hERG C-terminal isoform expression from the nonfunctional isoform to the functional isoform. Aim 3) To characterize a new mechanism of LQT2 in which premature termination followed by the reinitiation of translation results in the generation of N-terminally truncated hERG channels with altered gating properties. Aim 4) To create LQT2 patient-specific iPS cell lines and characterize LQT2 mutations in iPS cell-derived cardiomyocytes. This study will increase our knowledge of how LQT2 mutations lead to hERG channel dysfunction at the posttranscriptional and translational level. We believe that this work will have a sustained and significant impact on our understanding and treatment of long QT syndrome. PUBLIC HEALTH RELEVANCE: Long QT syndrome is a disease associated with delayed cardiac repolarization and prolonged QT intervals on the electrocardiogram, which can lead to ventricular arrhythmias and sudden death. The goal of present proposal is to elucidate how disease-causing mutations lead to hERG channel dysfunction and develop a novel approach to increase hERG channel function. The knowledge gained from this study will strengthen our understanding of the pathogenesis of hERG mutations in long QT syndrome and provide information directed towards the development of new therapeutic strategies for long QT syndrome.